<header>
    两向量的数量积
</header>
<p>
    <span class="title">
        定义
    </span>
    两个向量<code>["vector","a"]</code>和<code>["vector","b"]</code>的模和它们的夹角的余弦的乘积叫做向量
    <code>["vector","a"]</code>和<code>["vector","b"]</code>的数量积（也称内积），记做
    <code>["vector","a"]</code>·<code>["vector","b"]</code>或
    <code>["vector","a"]</code><code>["vector","b"]</code>，即
    <span class="oneline">
        <code>["vector","a"]</code>·<code>["vector","b"]</code> =
        |<code>["vector","a"]</code>||<code>["vector","b"]</code>|
        cos∠(<code>["vector","a"]</code>,<code>["vector","b"]</code>)
    </span>
</p>
<h2>
    一些相关定理
</h2>
<p>
    <span class="title">
        定理
    </span>
    设<code>["vector","a"]</code>=
    x1<code>["vector","i"]</code> +
    y1<code>["vector","j"]</code> +
    z1<code>["vector","k"]</code>，
    <code>["vector","b"]</code>=
    x2<code>["vector","i"]</code> +
    y2<code>["vector","j"]</code> +
    z2<code>["vector","k"]</code>，
    那么
    <span class="oneline">
        <code>["vector","a"]</code>·<code>["vector","b"]</code> = x1×x2 + y1×y2 + z1×z2
    </span>
</p>